The invention relates to superconducting conductors such as composite superconductors.
Superconductors are generally classified as either low or high temperature superconductors. An important property of the superconductor is the disappearance of its electrical resistance when it is cooled below its critical temperature Tc. For a given superconductor operating below Tc, a maximum amount of currentxe2x80x94referred to as the critical current (Ic) of the superconductorxe2x80x94exists which can be carried by the superconductor at a specified magnetic field and temperature below its critical temperature (Tc) and critical magnetic field (Hc) Under these conditions, any current in excess of Ic causes the onset of resistance in the superconductor.
The transition characteristics of low temperature superconducting (LTS) and high temperature superconducting (HTS) materials are quite different. In particular, for an LTS material, the transition between superconducting and non-superconducting occurs rather abruptly. This transition characteristic has made the use of LTS materials attractive in current limiting applications. Such LTS current limiting devices limit the flow of excessive current in electrical systems caused by, for example, short circuits, lightning strikes, or common power fluctuations. However, because the transition temperature of LTS materials is relatively low (i.e., generally about Tc=4 K to 30 K), the cost associated with cooling such materials to temperatures below their Tcs is high. Moreover, the cryogens used to achieve such low temperature add substantial cost and complexity, thereby lowering reliability of devices using such LTS materials, and must be handled with great care.
On the other hand, the transition characteristic of an HTS material is generally not abrupt. Rather, upon reaching its critical temperature or critical current value, the superconductivity of the HTS material gradually diminishes as its temperature or current flow rises until it reaches a temperature or current value at which the material no longer has any superconductivity characteristic.
Because of the significant non-linearities in the current-to-voltage (I-V) characteristics of both HTS and LTS materials over a range which includes both superconducting and non-superconducting states, small inhomogeneities in the critical current of the superconducting conductor, caused for example, by localized differences in diameter or composition, can lead to an inhomogeneous voltage distribution along the superconducting conductor, and localized thermal destruction of the superconducting conductor at the hot spots in this distribution.
The invention features an electrical conductor well-suited for superconducting current-limiting applications. The electrical conductor includes an elongated superconductor member including a superconducting material extending along its length. In one aspect, the superconductor member is a composite having superconducting material and a non-superconducting, electrically conductive matrix material. In another aspect, the superconductor member is a bulk superconducting material. In yet another aspect, the superconductor member is a composite having superconducting material mechanically supported by an substantially non-conductive matrix material. The electrical conductor is configured to control the manner in which the superconductor member transitions from its superconducting state to its non-superconducting (i.e., normal) state due to, for example, a fault current condition.
In a general aspect of the invention, the electrical conductor includes a thermal conductor attached to and along the length of the superconductor member, and an electrically-insulative material disposed between the thermal conductor and length of superconductor member, the electrically-insulative material having a thickness for allowing heat from the superconductor member to be conveyed to the thermal conductor.
In another aspect of the invention, a superconducting current-limiting device includes a support structure (e.g., an inner support tube) and the above-described electrical conductor disposed on the support structure.
In general, the superconductor member serves to carry the current during non-fault conditions with low electrical loss. In a preferred embodiment, a superconducting composite having the superconducting material and metal matrix, the matrix can provide an alternate current path to compensate for localized inhomogeneities in the superconductor during non-fault conditions. On the other hand, during a fault condition, the member carries the fault current with a high resistance for short periods of time. The thermal conductor serves as a thermal stabilizer and a heat sink during transient fault current events. In general, the thermal conductor prevents the superconducting material from getting too hot during a fault condition. The electrical conductor must not get too hot for two reasons: 1) it must have a quick thermal recovery characteristic, and 2) rapid large changes in temperature can cause thermal stresses sufficient to destroy the superconducting material. The thermal conductor is directly or indirectly thermally connected to a cooling channel. The cooling channel can be a liquid cryogen bath, gas cooling, or a conduction-cooling path (i.e., copper or alumina).
The electrically insulative material acts as a small thermal and electrical barrier having a two fold purpose. First, the insulative material prevents current transfer from the superconductor member into the thermal conductor. Second, the insulative material causes a delay in the heat transfer between superconductor member and the thermal barrier so that the superconducting material can completely revert to its normal, non-superconducting state.
The electrically-insulative material provides a thermal barrier that reduces the time needed for the superconductor to transition from a superconducting state to a non-superconducting state, thereby reducing the amount of current flowing through the superconducting material of the member and lowering the total current flowing through the electrical conductor. As a result, possible damage to the superconductor member caused by the excessive current flow and mechanical stresses (e.g., hoop stresses) is minimized.
Embodiments of either of the aspects of the invention may include one or more of the following features.
The superconducting material is in the form of a plurality of superconducting filaments, the superconducting filaments surrounded by a non-superconducting, metal matrix (e.g., silver or silver alloy). Alternatively, the superconducting material is in monolithic form as a coating on a non-conductive, or preferably a conductive, ceramic buffer layer supported on a metal substrate (e.g., a nickel, copper or aluminum alloy).
The thermal conductor is formed of spaced-apart thermally-conductive elements. The electrically-insulative material and the thermal conductor, together, form a first laminate. A second laminate is disposed on the first laminate and includes a second sheet of electrically-insulative material disposed on the first plurality of the thermal conductive elements of the first laminate and a second thermal conductor (preferably formed of a second plurality of spaced-apart thermally-conductive elements) attached to the second electrically-insulative material.
The thermally-conductive elements of the first laminate and the thermally-conductive elements of the second laminate are offset from each other relative to the length of the superconductor member. As a result, the electrical conductor has increased mechanical strength, particularly in wound structures.
In certain embodiments, the first laminate is attached to a first side of the length of the superconductor member and the second laminate is attached to a second, opposite side of the length of the superconductor member. The superconductor member in this form is xe2x80x9csandwichedxe2x80x9d between two stabilizer members, thereby providing improved thermal stability and mechanical strength.
The superconductor member is in the form of a tape having a thickness in a range between about 25 to 300 microns. The superconducting material is formed of a high temperature, anisotropic superconducting material.
The thermal conductor may be formed from a metal, a metal alloy or an electrical insulator with good thermal conductivity such as a highly mineral filled resin, but preferably is formed of stainless steel. Each of the thermally conductive elements have a thickness in a range between about 100 and 1,000 microns. The electrically-insulative material is formed of a glass epoxy or other material, such as lacquer cured by conventional or UV systems, thermoplastics, epoxy resins, or composites (i.e. glass fiber/resin composites or impregnated papers), capable of serving as an insulator at cryogenic temperatures. In certain embodiments, the electrically-insulative material is a polyimide material having a thickness in a range between about 5 and 50 microns.
In an alternative embodiment, the superconducting material is in monolithic form with the non-superconducting matrix in intimate contact with the superconducting material.
In still another aspect of the invention, a method of fabricating a superconducting composite includes attaching a thermal conductor along a length of high temperature superconductor, and providing an electrically-insulative material between the thermal conductor and the length of high temperature superconductor, the electrically-insulative material having a thickness which allows heat from the high temperature superconductor to be conveyed to the thermal conductor. In embodiments of this aspect of the invention, a first laminate of the thermal conductor and the electrically-insulative material is provided. Further, in certain embodiments, providing the thermal conductor elements along the length of superconductor composite and providing the electrically-insulative material includes removing opposing side portions of an insulated conductive wire.
Other advantages and features will become apparent from the following description and the claims.